1. Field of the Invention
The present invention relates, in general, to a heat exchanger of a storage type boiler, capable of preventing corrosion of the heat exchanger by greatly reducing the generation of condensate water on the outer surface of the heat exchanger in the storage type boiler.
2. Discussion of Background Information
A boiler is defined as an apparatus for boiling water in order to supply hot water to a heating facility or a bath. In brief, the boiler is a device for boiling water, and can be divided into two types: one for industrial use and the other for household use.
Typically, a household boiler is a low-capacity boiler used below boiling point, whereas an industrial boiler is a high-capacity boiler used above boiling point. Other, additional devices are required for use above the boiling point, and thus increase the size of equipment. For this reason, most household boilers using relatively little thermal energy can heat water to below the boiling point.
Further, boilers are classified as condensing type boilers and non-condensing type boilers, according to whether or not condensing is carried out.
The condensing type boiler emits thermal energy in a process in which steam, generated by the combustion of gas, is converted into water (H2O) upon contact with lower temperature objects or air, and employs a system that is designed to recover the thermal energy to increase thermal efficiency. The condensing type boiler can obtain thermal efficiency between 103% and 108% on the basis of a net caloric value, and thus is a type of boiler that reduces fuel costs remarkably. This condensing type boiler can have high thermal efficiency, but cannot prevent the generation of condensate water. Hence, the condensing type boiler is produced using material that has high corrosion resistance on heat transfer surfaces of the boiler. Typically, material having high corrosion resistance has low heat transfer efficiency, so that the heat transfer area of the boiler must be increased in order to guarantee heat transfer efficiency, and thus an increase in the size of the boiler must be tolerated.
The non-condensing type boiler, compared to the condensing type boiler, is free from condensation, so it has an advantage in that it can be reduced in size by using material characterized by relatively efficient heat transfer. However, because materials exhibiting relatively good heat transfer efficiency are not efficiently resistant to the corrosive effects of condensate water, the non-condensing type boiler shows a tendency toward a rapid decrease in lifespan due to the condensate water generated when operation is commenced.
Ideally, the non-condensing type boiler does not generate any condensate water.
However, this is impossible in practice because a boiler is not operated at all times. Therefore, an alternative construction in which relatively less condensate water is generated is necessary in order to prolong the lifespan of the boiler.
FIG. 1 illustrates the structure of a regular heat exchanger in a conventional storage type boiler. The regular heat exchanger is provided with a combustion chamber to which a burner is attached at an upper or lower portion thereof, and a plurality of fire tubes. An exhaust gas that is burned transfers heat to water through the wall of each fire tube while the gas passes through the plurality of fire tubes, and is discharged to the outside through an exhaust flue (not shown). The burned exhaust gas contains a large quantity of water vapor (H2O), which is condensed into water when contacting an object that is cooler than a dew point temperature.
The dew point temperature is dependent on the percentage of water vapor in the exhaust gas. In the state of ordinary combustion, the dew point temperature is in the range of 40° C. to 47° C. in the case of oil fuel, and of 50° C. to 57° C. in the case of gas fuel. If the heating water is at an inflow temperature of 20° C. and an outflow temperature of 40° C. when heated by the heat exchanger, the water in the heat exchanger has an overall temperature under the dew point temperature, and thus a condensation phenomenon rapidly occurs on the wall of each fire tube. In the case in which a user has been absent, when operation of the boiler again is initiated, water is mostly circulated at a temperature below 20° C. Hence, in the case in which this condensation phenomenon is repeatedly generated, moisture, when condensed, reacts with sulfur oxides (in the case of oil fuel) or nitrogen oxides (in the case of gas fuel), so that acid condensate water between pH 2 and pH 4 is formed. As the temperature in the boiler is gradually increased, the moisture of the acid condensate water evaporates, leaving only the sulfur oxides or the nitrogen oxides. As the condensation phenomenon is repeated, the acid condensate gradually adheres to the surface of the boiler. The acid condensate causes the heat exchanger to gradually corrode and become less durable. Ultimately, due to the acid condensate water, the boiler can no longer be used.
Typical materials for the heat exchanger include iron, copper, and the like. These metals are very vulnerable to corrosion caused by the condensate water. Hence, when the boiler is used for some time, corrosion occurs. Of course, the heat exchanger can be produced using a special material such as aluminum alloy or stainless steel, so as to withstand the condensate water. In the case in which such a special material is used, production and operation are difficult, and the size, weight and cost of the product are increased. Further, these special materials have relatively low heat transfer efficiency, compared to copper.
For this reason, a heat exchanger capable of preventing damage as well as having improved durability despite being made of typical material, because it prevents or minimizes the condensation phenomenon, is required.